apz.pan_repaint_interval is set to a glacial 250ms. We should make it faster. BenWa tested with 30ms and it was nice. Kevin says at https://bugzilla.mozilla.org/show_bug.cgi?id=967884#c24 that at 100ms it's nice too.
I'm going to make it 40ms to keep in in sync with the asyncscroll interval.

Sorry this title feels misleading to people who haven't followed this conversation.
The tradeoff where making here is that the main thread will be idle less which means we're be using our CPU more effectively at painting but if we spend more time doing paint overhead (displaylist, buffer rotation) then we're worse off with this change although I doubt it will be.